1. Field of the Invention
The present invention relates to a fuel cell. More particularly, the present invention relates to a PEFC (Polymer Electrolyte Fuel Cell) where a drying-up and a flooding of the fuel cell are prevented.
2. Description of Related Art
A PEFC (Polymer Electrolyte Fuel Cell) apparatus includes individual fuel cells. Each fuel cell includes a membrane-electrode assembly (MEA) and a separator. The MEA includes an electrolyte membrane and a pair of electrodes disposed on opposite sides of the electrolyte membrane. The pair of electrodes include an anode provided on one side of the membrane and constructed of a first catalyst layer and a first diffusion layer, and a cathode provided on the other side of the membrane and constructed of a second catalyst layer and a second diffusion layer. The separator has a passage formed therein for supplying fuel gas (hydrogen) to the anode and for supplying oxidant gas (oxygen, usually, air) to the cathode. A plurality of fuel cells are piled to construct a module. A number of modules are piled, and electrical terminals, electrical insulators, and end plates are disposed at opposite ends of the pile of modules to construct a stack of fuel cells. After tightening the stack of fuel cells between the opposite end plates in a fuel cell stacking direction, the end plates are coupled to the fastening member (for example, a tension plate) extending in a fuel cell stacking direction outside the pile of fuel cells by bolts extending perpendicularly to the fuel cell stacking direction.
In the PEFC, at the anode, hydrogen is changed to positively charged hydrogen ions (i.e., protons) and electrons. The hydrogen ions move through the electrolyte to the cathode where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of the adjacent MEA and move to the cathode of the instant MEA through a separator) to form water as follows:At the anode: H2→2H++2e−At the cathode: 2H++2e−+(½)O2→H2O
In order that the hydrogen ions move through the electrolyte, the electrolyte membrane has to be aqueous. If the concentration of water in the electrolyte membrane decreases, the electric resistance of the electrolyte membrane increases, resulting in a decrease in an output voltage and a decrease in an output power. If the electrolyte membrane dries out too much, it cannot operate as an electrolyte.
Usually, the composition and the structure of the MEA are uniform in a cell plane. However, a gas concentration and a flow amount of the reactant gas and a partial pressure of the water vapor differ greatly between an upstream side and a downstream side in the reactant gas flow direction. As a result, a drying-up (drying-out) is apt to occur at the upstream side, and a flooding is apt to occur at the downstream side. Accordingly, it is difficult to cause the entire area of the fuel cell to operate stably. The drying-up will cause a decrease in the aqueous concentration of the electrolyte membrane, and the flooding will cause an insufficiency of gas supply and will reduce the performance of the cell.
Japanese Patent Publication No. HEI 6-267562 discloses a fuel cell where the structure of the diffusion layer of the electrode of the fuel cell varies in the gas flow direction so that a product water is easily removed and a flooding is suppressed.
However, the flooding prevention structure is applied to the diffusion layer only in the fuel cell of Japanese Patent Publication No. HEI 6-267562. The structure of the catalyst layer of the electrode, which is closer to the electrolyte membrane than the diffusion layer, is constant in the cell plane. Therefore, there is a room to further improve the drying-up and flooding prevention characteristic of the cell.